Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. A conventional PVD reactor 10 is illustrated schematically in cross section in FIG. 1, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor 10 includes a vacuum chamber 12 sealed to a PVD target 14 composed of the material, usually a metal, to be sputter deposited on a wafer 16 held on a heater pedestal 18. A shield 20 held within the chamber protects the chamber wall 12 from the sputtered material and provides the anode grounding plane. A selectable DC power supply 22 negatively biases the target 14 to about -600 VDC with respect to the shield 20. Conventionally, the pedestal 18 and hence the wafer 16 are left electrically floating.
A gas source 24 supplies a sputtering working gas, typically the chemically inactive gas argon, to the chamber 12 through a mass flow controller 26. In reactive metallic nitride sputtering, for example, of titanium nitride, nitrogen is supplied from another gas source 27 through its own mass flow controller 26. Oxygen can also be supplied to produce oxides such as Al.sub.2 O.sub.3. The gases can be admitted to the top of the chamber, as illustrated, or at its bottom, either with one or more inlet pipes penetrating the bottom of the shield or through the gap between the shield 20 and the pedestal 18. A vacuum system 28 maintains the chamber at a low pressure. Although the base pressure can be held to about 10.sup.-7 Torr or even lower, the pressure of the working gas is typically maintained at between about 1 and 1000 mTorr. A computer-based controller 30 controls the reactor including the DC power supply 22 and the mass flow controllers 26.
When the argon is admitted into the chamber, the DC voltage between the target 14 and the shield 20 ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target 14. The ions strike the target 14 at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target 14. Some of the target particles strike the wafer 16 and are thereby deposited on it, thereby forming a film of the target material. In reactive sputtering of a metallic nitride, nitrogen is additionally admitted into the chamber 12, and it reacts with the sputtered metallic atoms to form a metallic nitride on the wafer 16.
To provide efficient sputtering, a magnetron 32 is positioned in back of the target 14. It has opposed magnets 34, 36 creating a magnetic field within the chamber in the neighborhood of the magnets 34, 36. The magnetic field traps electrons and, for charge neutrality, the ion density also increases to form a high-density plasma region 38 within the chamber adjacent to the magnetron 32. The magnetron 32 is usually rotated about the center of the target 14 to achieve full coverage in sputtering of the target 14. The form of the magnetron is a subject of this patent application, and the illustrated form is intended to be only suggestive.
The advancing level of integration in semiconductor integrated circuits has placed increasing demands upon sputtering equipment and processes. Many of the problems are associated with contact and via holes. As illustrated in the cross-sectional view of FIG. 2, via or contact holes 40 are etched through an interlevel dielectric layer 42 to reach a conductive feature 44 in the underlying layer or substrate 46. Sputtering is then used to fill metal into the hole 40 to provide inter-level electrical connections. If the underlying layer 46 is the semiconductor substrate, the filled hole 40 is called a contact; if the underlying layer is a lower-level metallization level, the filled hole 40 is called a via. For simplicity, we will refer hereafter only to vias. The widths of inter-level vias have decreased to the neighborhood of 0.25 .mu.m and below while the thickness of the inter-level dielectric has remained nearly constant at around 0.7 .mu.m. As a result, the via holes in advanced integrated circuits have increased aspect ratios of three and greater. For some technologies under development, aspect ratios of six and even greater are required.
Such high aspect ratios present a problem for sputtering because most forms of sputtering are not strongly anisotropic, a cosine dependence off the vertical being typical, so that the initially sputtered material preferentially deposits at the top of the hole and may bridge it, thus preventing the filling of the bottom of the hole and creating a void in the via metal.
It has become known, however, that deep hole filling can be facilitated by causing a significant fraction of the sputtered particles to be ionized in the plasma between the target 14 and the pedestal 18. The pedestal 18 of FIG. 1, even if it is left electrically floating, develops a DC self-bias, which attracts ionized sputtered particles from the plasma across the plasma sheath adjacent to the pedestal 18 and deep into the hole 40 in the dielectric layer 42. The effect can be accentuated with additional DC or RF biasing of the pedestal electrode 18 to additionally accelerate the ionized particles extracted across the plasma sheath towards the wafer 16, thereby controlling the directionality of sputter deposition. The process of sputtering with a significant fraction of ionized sputtered atoms is called ionized metal deposition or ionized metal plating (IMP). Two related quantitative measures of the effectiveness of hole filling are bottom coverage and side coverage. As illustrated schematically in FIG. 2, the initial phase of sputtering deposits a layer 50, which has a surface or blanket thickness of s.sub.1, a bottom thickness of s.sub.2, and a sidewall thickness of s.sub.3. The bottom coverage is equal to s.sub.2 /s.sub.1, and the sidewall coverage is equal to s.sub.3 /s.sub.1. The model is overly simplified but in many situations is adequate.
One method of increasing the ionization fraction is to create a high-density plasma (HDP), such as by adding an RF coil around the sides of the chamber 12 of FIG. 1. An HDP reactor not only creates a high-density argon plasma but also increases the ionization fraction of the sputtered atoms. However, HDP PVD reactors are new and relatively expensive, and the quality of the deposited films is not always the best. It is desired to continue using the principally DC sputtering of the PVD reactor of FIG. 1.
Another method for increasing the ionization ratio is to use a hollow-cathode magnetron in which the target has the shape of a top hat. This type of reactor, though, runs very hot and the complexly shaped targets are very expensive.
It has been observed that copper sputtered with either an inductively coupled HDP sputter reactor or a hollow-cathode reactor tends to form an undulatory copper film on the via sidewall, and further the deposited metal tends to dewet. The variable thickness is particularly serious when the sputtered copper layer is being used as a seed layer of a predetermined minimum thickness for a subsequent deposition process such as electroplating to complete the copper hole filling.
A further problem in the prior art is that the sidewall coverage tends to be asymmetric with the side facing the center of the target being more heavily coated than the more shielded side facing a larger solid angle outside the target. Not only does the asymmetry require excessive deposition to achieve a seed layer of predetermined minimum thickness, it causes cross-shaped trenches used as alignment indicia in the photolithography to appear to move as the trenches are asymmetrically narrowed.
Another operational control that promotes deep hole filling is chamber pressure. It is generally believed that lower chamber pressures promote hole filling. At higher pressures, there is a higher probability that sputtered particles, whether neutral or ionized, will collide with atoms of the argon carrier gas. Collisions tend to neutralize ions and to randomize velocities, both effects degrading hole filling. However, as described before, the sputtering relies upon the existence of a plasma at least adjacent to the target. If the pressure is reduced too much, the plasma collapses, although the minimum pressure is dependent upon several factors.
The extreme of low-pressure plasma sputtering is sustained self-sputtering (SSS), as disclosed by Fu et al. in U.S. patent application Ser. No. 08/854,008, filed May 8, 1997. In SSS, the density of positively ionized sputtered atoms is so high that a sufficient number are attracted back to the negatively biased target to resputter more ionized atoms. Under the right conditions for a limited number of target metals, the self-sputtering sustains the plasma, and no argon working gas is required. Copper is the metal most prone to SSS, but only under conditions of high power and high magnetic field. Copper sputtering is being seriously developed because of copper's low resistivity and low susceptibility to electromigration. However, for copper SSS to become commercially feasible, a full-coverage, high-field magnetron needs to be developed.
Increased power applied to the target allows reduced pressure, perhaps to the point of sustained self-sputtering. The increased power also increases the ionization density. However, excessive power requires expensive power supplies and increased cooling. Power levels in excess of 20 to 30 kW are considered infeasible in a commercial environment. In fact, the pertinent factor is not power but the power density in the area below the magnetron since that is the area of the high-density plasma promoting effective sputtering. Hence, a small, high-field magnet would most easily produce a high ionization density. For this reason, some prior art discloses a small circularly shaped magnet. However, such a magnetron requires not only rotation about the center of the target to provide uniformity, but it also requires radial scanning to assure full and fairly uniform coverage of the target. If full magnetron coverage is not achieved, not only is the target not efficiently used, but more importantly the uniformity of sputter deposition is degraded, and some of the sputtered material redeposits on the target in areas that are not being sputtered. Furthermore, the material redeposited on unsputtered areas may build up to such a thickness that it is prone to flake off, producing severe particle problems. While radial scanning can potentially avoid these problems, the required scanning mechanisms are complex and generally considered infeasible in a production environment.
One type of commercially available magnetron is kidney-shaped, as exemplified by Tepman in U.S. Pat. No. 5,320,728. Parker discloses more exaggerated forms of this shape in U.S. Pat. No. 5,242,566. As illustrated in plan view in FIG. 3, the Tepman magnetron 52 is based on a kidney shape for the magnetically opposed pole faces 54, 56 separated by a circuitous gap 57 of nearly constant width. The pole faces 54, 56 are magnetically coupled by unillustrated horseshoe magnets bridging the gap 57. The magnetron rotates about a rotational axis 58 at the center of the target 14 and near the concave edge of the kidney-shaped inner pole face 54. The convexly curved outer periphery of the outer pole face 56, which is generally parallel to the gap 57 in that area, is close to the outer periphery of the usable portion if the target 14. This shape has been optimized for high field and for uniform sputtering but has an area that is nearly half that of the target. It is noted that the magnetic field is relatively weak in areas separated from the pole gap 57.
For these reasons, it is desirable to develop a small, high-field magnetron providing full coverage so as to promote deep hole filling and sustained copper self-sputtering.